Mechanisms of metabolism other than those mediated by the mixed fraction oxidases may be important in activating certain chemicals to their ultimate carcinogenic form. Recent evidence suggests that benzidine dyes are cleaved by intestinal bacteria, thereby liberating the parent benzidine and its congeners. There is evidence implicating peroxidases in the metabolism of benzidine dyes. Prostaglandin H synthetase will be used to activate compounds in the mouse lymphma forward mutation assay. The possible mechanisms responsible for the formation of mutagenic metabolites induced by various peroxidase enzyme systems are being investigated. The use of selective inhibitors of the peroxidase are being used to aid in the elucidation of these mechanisms. Studies include the identification of metabolites induced by this activation system.